There is not one specific recommendation regarding image files. In general you should save images in the best possible resolution and lossless compression. However, this does hardly make any sense if image files are already available in compressed form.
- TIFF: classic long-term archiving format, but only uncompressed; as TIFF 6.0, Part 1: Baseline TIFF. Other TIFFs are not suitable for long-term preservation.
- JPEG2000 and PNG: both recommended for lossless compression. JPEG2000 is also used for PDF/A-2.
- jpg: Acceptable for many already existing files in this format due to the high distribution.
- Not desirable: Not widely distributed and proprietary formats, e.g. RAW, Photoshop PSD, PCD, Encapsulated PostSrcipt, FlashPix.
- Other formats? Please consult with us.
Note: There are a number of softwares that allow batch conversion of multiple files, e.g. Irfanview.